Power consumption is an important feature for mobile terminals using a battery power supply and also for mobile terminals using external power supply. Its importance increases with the continued growth of device populations such as for example the proliferation of Machine Type Communication (MTC) devices and also the deployment of devices in ever more demanding situations and environments. For example, for Machine To Machine (M2M) use cases, such as remote sensors that run on battery-power, there are significant cost implications for on-site exchange (or re-charge) of the batteries for a large device population and the battery lifetime may even determine the device's lifetime if it is not foreseen to re-charge or replace the battery.
Discussion within the 3rd Generation Partnership Project (3GPP) for mobile communications have resulted in numerous study and work items related to such problems. 3GPP Services and Systems Aspects (SA2) (architecture) group has discussed UE power consumption optimisation for energy saving and battery power consumption optimisation (UEPCOP) within the MTC work item. The main objective study is identified as:                Lower UE Power Consumption as per the service requirements defined clause in clause 7.1.1 of Technical Specification TS 22.368.In particular, 3GPP SA2 has already identified several proposals for UE power consumption optimization in Technical Report TR 23.887.        
One proposal outlined in TR23.887, at section 7.1.3.3, relates to employment of a Power Saving Mode/State (PSM) to be deployed by a mobile terminal for reducing its power consumption. That mode described is similar to a power-off mode, but the mobile terminal remains registered with the network and so there is no need to re-attach or re-establish Packet Data Network (PDN) connections. A mobile terminal in PSM is not immediately reachable for Mobile Terminating (MT) services. However, such a terminal in PSM can be made available for MT services but only for the period of an Active Time after a Mobile Originated (MO) event such as data transfer or signalling, e.g. after a periodic Tracking Area Update/Routing Area Update (TAU/RAU) procedure.
When the mobile terminal is use the PSM, a request is made for an Active Time value, and a request for a Periodic TAU/RAU Timer value, during Attach and TAU procedures. The mobile terminal and a network device such as a Mobile Management Entity/Serving GPRS Support Node MME/SGSN start the Active timer with the Active Time value that is allocated by the network when mobile terminal is transiting from a Connected mode to an Idle mode. When the Active timer expires, the mobile terminal deactivates its Access Stratum functions and enters PSM. In PSM, due to deactivation of the Access Stratum functions, the mobile terminal stops all idle mode procedures, but continues to run the periodic TAU/RAU timer. The mobile terminal will be arranged to resume Access Stratum functions and idle mode procedures before the periodic TAU/RAU timer expires for performing the periodic TAU/RAU procedure as applicable. Further, the mobile terminal may resume idle mode procedures and Access Stratum functions any time during PSM, for example e.g. for MO communications.
When the Active Timer value expires, the MME/SGSN can determine that the mobile terminal has entered PSM and so is not available for paging.
In PSM the mobile terminal is in Idle mode and the MME/SGSN will treat the mobile terminal as if registered, but not reachable. The Access Stratum functions of the mobile terminal are considered as deactivated during PSM.
An alternative Power Saving scenario is discussed at section 7.1.3.4 of TR23.887 and based on Attach/Detach procedures.
This proposal is based on the functionality that the mobile terminal attaches to the network to communicate and, after the communication session, the mobile terminal detaches from the network and switches off its 3GPP modem (MT) during a period to save mobile terminal power. However, the mobile terminal will periodically perform the Attach and Detach procedures.
Two particular examples are discussed and the first comprises Network Assisted Power Saving (NAPS) and is outlined at section 7.1.3.4.3.2 of TR23.887.
To summarise, many mobile terminals (e.g. MTC devices for vendor machines, electricity metering, weather sensors, etc) exhibit expected/scheduled communication patterns in which the device and Application server communicate at regular intervals only (e.g. once a day or once a week etc). There is therefore a possibility to save energy if the mobile terminal detaches in between these occasions, and in addition to the reduction in the usage of network resources. As a service, the operator may offer the Machine-to-Machine (M2M) service provider the possibility of to simplifying such regular communication. Based on agreement as to how often the communication needs to occur, the operator can pre-configure information in the network, and optionally the mobile terminal, for assisting regular attach/detach of the mobile terminal. The NAPS proposal provides a mechanism to enhance the existing Network Initiated Detach procedure for such scenarios. Also, this function allows the mobile terminal to propose a time for remaining attached (identified as User Equipment (UE) power saving active timer) and this allows the network to provide the mobile terminal with a new timer (UE power saving wakeup timer). The mobile terminal can then re-attach to the network when the timer has expired. As the mobile terminal detachment and re-attachment is performed by way of network-control, it is possible for mobile terminal to look to achieve both batter-power saving, and network signalling optimization, while retaining network control of when the mobile terminal should look to perform re-attachment.
The second example, referred to at section 7.1.3.4.3.3 of TR23.887 as Implicit Detach, is based on the coordinated (i.e. synchronized) transition from an attached state to a detached state in both the network and the mobile terminal but without explicit signalling. The Implicit Detach is performed soon after communication is completed in order to avoid long idle state periods as commonly arising, for example the periodic TAU/RAU period at UE, the reachable tinier+detach timer at the network, which can be in order of hours.
Generally, to ensure the coordinated implicit transition to a detached state, the configuration of two main parameters in the network and in the mobile terminal is required first a certain “implicit detach time” is identified and secondly an “event/point” when this implicit detach timer starts. The term “implicit detach time” refers to the time duration from a certain event (start point) until the mobile terminal and network implicitly transfer to detached state.
For implicit detach coordination with the network, and when a mobile terminal is configured for Implicit Detach, either the mobile terminal needs to indicate its Implicit Detach capability configuration to the network (in NAS signalling for example); or the MME can get the mobile terminal's Implicit Detach capability configuration from its subscription information (e.g. provisioned by the M2M service capability layer from the Service Capability Server (SCS).
In such configuration, the implicit detach time is either exchanged between the network and the mobile terminal, takes the value of the “grant time interval” or the “communication window” if time-controlled communication, or takes the value of the Periodic TAU timer.
In the latter case the network allocates the Periodic TAU timer by taking into account the subscription information or configuration.
Thus various solutions for mobile terminal power saving and optimization are proposed in TR23.887 and each solution has its specific characteristics and suits specific operating scenarios.
For example, the Power Saving Mode/State of section 7.1.3.3 permits long battery lifetime for stationary and moving UEs with infrequent communications needs. Although the UE is kept attached, the UE is not reachable alter active timer expiry until the next periodic TA/RA update. However, the UE can still be reached via SMS messages, which can be delivered when the UE wakes up as it remains active for the active time after contacting the network.
However, the Power Saving Mode/State can add signalling load due to the polling of the network with the periodic updates even when there is no uplink or downlink data to exchange. This makes the Power Save State/Mode not suitable for scenarios where the mobile terminal needs to transmit very rarely, such as for example, once a day or even more infrequently (i.e. utility meters, some e-health devises etc).
The Attach/Detach solutions of section 7.1.3.4 involve detachment of the mobile terminal detached and so does not generate signaling load onto the system (e.g. RAU/TAU). Also, the SGSN/MME does not need to retain the mobile terminal's context and this can serve to free-up many network-side resources. Also, with the implicit detach approach a further saving in the signaling can be achieved as there is no need for explicit detach signaling. The mobile terminal and the network can then perform a coordinated implicit detach.
It is clear that the attach/detach approach would suit MTC scenarios where the mobile terminal communicates very infrequently and it is tolerant to paging (such as for example utility meters etc.). Indeed, this is recognized in Technical Specification TS22.368, at section 7.1.1 where it is noted that “MTC Devices may or may not be kept attached to the network when not communicating, depending on operator policies and MTC Application requirements.”